I won't get too deep into the Halo 2/3 thing, since we've done that so many times, but one thing I think Halo 3 got exactly right was the vehicle sandbox, and how they balanced infantry against it. It's just so head and shoulders above Halo 2 (and Reach, for that matter). It's one of the main reasons I love that game so much. On any given map, I could either romp around with the 'Hog or the Chopper (<3), or go on a vehicle killing mission. So many tools to take them down with, but they were still formidable. It was such a huge step forward from Halo 2 (tracking rocket launcher says hi), and Reach such a step back from it.